1. Field of the Invention
The present invention relates to a semiconductor laser having excellent noise characteristics and a stable single lateral mode, and a method for producing the same.
2. Description of the Related Art
Semiconductor lasers which oscillate laser light in visible regions have applications to such devices as laser beam printers and light sources for optical information processing devices such as optical disk apparatuses, and therefore tend to have increased importance these days. Materials of an (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P type, in particular, are attracting much attention because they can easily be lattice-matched to GaAs, which is an excellent material for substrates, and can oscillate laser light having a wavelength in the range of 0.56 .mu.m to 0.69 .mu.m by varying the Al mole fraction x.
Hereinafter, a conventional semiconductor laser having a doublehetero structure, which oscillates laser light of wavelengths pertaining to red regions, will be described with reference to FIG. 1.
As shown in FIG. 1, this semiconductor laser includes an n-GaAs substrate 1, an n-GaAs buffer layer 2, an n-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 3, a Ga.sub.0.5 In.sub.0.5 P active layer 4, a p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 5, p-Ga.sub.0.5 In.sub.0.5 P layer 6, an n-GaAs current confining layer 8, and a p-GaAs contact layer 9 formed in this order. A p-side electrode 10 and an n-side electrode 11 are formed, respectively, upon the p-GaAs contact layer 9 and upon the bottom face of the substrate 1.
The above-mentioned semiconductor laser can be fabricated by a crystal growing method such as a metal organic vapor phase epitaxy (MOVPE) method. By the use of such a crystal growing method, the n-GaAs buffer layer 2, the n-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 3, the Ga.sub.0.5 In.sub.0.5 P active layer 4, the p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 5, and the p-Ga.sub.0.5 In.sub.0.5 P layer 6 are grown upon the n-GaAs substrate 1 in this order. Next, by photolithography and etching, the p-Ga.sub.0.5 In.sub.0.5 P layer 6 and a portion of the p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 5 are etched so as to form a stripe-shaped ridge having a trapezoidal cross-section in the p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 5. Then, by an MOVPE method or the like, the n-GaAs current confining layer 8 is selectively grown on the outside of the stripe, and the p-GaAs contact layer 9 is further grown.
In accordance with a semiconductor laser of the above-mentioned configuration, a current can be confined into a relatively narrow stripe-shaped region of the Ga.sub.0.5 In.sub.0.5 P active layer 4 by the n-GaAs current confining layer 8. Moreover, during the etching of a portion of the p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 5 into the stripe-shaped ridge having a trapezoidal cross section, the effective refraction indices inside and outside the stripe-shaped ridge can be ensured to have a difference which satisfies requirements of a single lateral mode by optimizing the height and width of the trapezoid. As a result, light can be effectively contained in a predetermined region of the Ga.sub.0.5 In.sub.0.5 P active layer 4, the predetermined region being under the stripe-shaped ridge of the p-(Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 5. A typical width of the stripe-shaped ridge is about 5 .mu.m.
However, according to the above-mentioned configuration, the longitudinal mode tends to be stabilized as a single mode. As a result, noise is likely to occur when the semiconductor laser is applied to an optical disk or the like. The noise is categorized into a returned light-induced noise, a mode-hopping noise, and the like, as described below:
The returned light-induced noise is defined to be noise that occurs when laser light emitted from a semiconductor laser is reflected by a lens, an optical disk, or the like so as to be fed back into a cavity of the semiconductor laser. Laser light emitted from a semiconductor laser oscillating in a single longitudinal mode has a high degree of coherency. Therefore, depending on the positions of reflecting surfaces such as a lens and an optical disk and/or intensity of the reflected laser light, the optical output power of the semiconductor laser may fluctuate, and the spectrum of the optical output of the semiconductor laser may fluctuate (i.e. mode hopping), whereby noise is generated.
The mode-hopping noise is most likely to occur when the thermal circumstances of the semiconductor laser change. Specifically, the mode-hopping noise may occur when a given order of longitudinal mode shifts to a next order (i.e. when a mode hopping occurs) due to change in the thermal circumstances, resulting in repetition of randomly alternating oscillations of different orders of longitudinal modes and/or fluctuation of optical output power due to different optical output powers of the longitudinal modes.
The returned light-induced noise and the mode-hopping noise equally constitute a problem in practice, e.g. when applied to an optical disk, because it may degrade the quality of the reproduced sound and image information carried on the optical disk.
In view of the above problems, a configuration for improving the noise characteristics is proposed in Japanese Laid-Open Patent Publication No. 62-39084. This semiconductor laser includes, as is shown in FIGS. 2A to 2C, an n-GaAs substrate 201, an internal current confining layer 203, a buffer layer 204, an n-cladding layer 205, an active layer 206, a p-cladding layer 207, a p-GaAs layer 208, a p-side electrode 209, and an n-side electrode 210. This semiconductor laser is composed essentially of AlGaAs.
As shown in FIG. 2C, in region B (including line B-B' in FIG. 2A) in the vicinity of an end face of a cavity, the semiconductor laser has a bent waveguide configuration. On the other hand, as shown in FIG. 2B, the stripe width is made large in region A (including line A-A' in FIG. 2B) in a central portion of the cavity. The internal current confining layer 203 ensures that an injected current is led to a predetermined region of the active layer 206. The dimensions of the semiconductor laser are as follows: the cavity length is about 300 .mu.m; the length of region B is about 30 .mu.m; the width of region B is about 2.5 .mu.m; the length of region A is about 240 .mu.m; and the width of region A is about 8 .mu.m.
In the above-described semiconductor laser, a multitude of longitudinal modes are realized in region A, so that the noise characteristics of the semiconductor laser are improved.
However, in cases where an internal current confining layer is provided on the n-GaAs substrate side of the active layer, as in the configuration shown in FIGS. 2A to 2C, the electron mobility is so large that an injected current may spread in an excessively large area along a direction parallel to a direction in which each layer of the semiconductor laser extends (hereinafter, this direction will be referred to as the "horizontal direction"). This makes it difficult to inject the current into the predetermined region of the active layer 206. As a result, the lateral mode may become unstable, and the driving current for the semiconductor laser may increase.
On the contrary, if the current confining layer 203 is provided on the p-cladding layer 207 side of the active layer 206, the heterojunction for holes between the p-cladding layer 207 and the p-GaAs layer 208 ( which is provided so that the p-electrode 209 can be provided thereon) is so large that holes injected from the p-side electrode 209 may spread in an excessively large area in region A. As a result, the driving current for the semiconductor laser may increase, and the distribution of the injected holes may fluctuate as with the changes in the thermal circumstances. These problems are particularly eminent in cases where an AlGaInP type material is used for the configuration shown in FIG. 17, since the heterojunction in the valence band thereof is larger than that of an AlGaAs type material. For example, in the case of a red-light semiconductor laser capable of oscillating in the 670 nm band, the injected holes spread about 50 .mu.m along the horizontal direction, so that the driving current increases, indicative of poor practicality. Moreover, because of the presence of region A, where the stripe width is made large so as to have little control over the lateral mode, stable lateral-mode optical output power cannot be realized at a high level.